CN1998084A - Tunnel junction barrier layer including dilute magnetic semiconductor with spin sensitivity - Google Patents

Abstract

A magnetic tunnel junction having a tunneling barrier layer, wherein the tunneling barrier layer comprises a diluted magnetic semiconductor having spin sensitivity. The magnetic tunnel junction according to the present invention may comprise a bottom lead coupled to a bottom electrode, the bottom electrode being coupled to a diluted magnetic semiconductor, the diluted magnetic semiconductor being coupled to a top electrode, the top electrode being coupled to a top lead, wherein the bottom electrode is non-magnetic. The invention also provides various components and computers employing magnetic tunnel junctions according to the invention.

Description

Tunnel junction barrier layer including dilute magnetic semiconductor with spin sensitivity

technical field

The present invention relates to magnetic tunnel junction (MTJ) devices for spin-sensitive electronic and optical applications. These applications include nonvolatile magnetic random access memory (MRAM), magnetoresistive read heads for disk drives, spin valve/magnetic tunnel transistors, ultrafast optical switches, and light emitting devices with polar modulated outputs. Other applications that may include the invention as a subsystem include logic devices with variable logic functions and quantum computers. In particular, the present invention uses a tunnel barrier with spin filtering function to improve the properties and performance of MTJs.

Background technique

Magnetic tunnel junctions (MTJs) are devices that use the magnetoresistance effect to modulate electrical conduction. An MTJ device consists of two ferromagnetic electrodes separated by an insulating barrier layer made thin enough to allow quantum mechanical tunneling of charge carriers between the electrodes (Fig. 1(a )). In the electrodes, the charge carriers are spin polarized due to magnetic properties. Most of the spins are aligned with the magnetization direction of each electrode respectively. Since the tunneling process is spin-dependent, the magnitude of the tunneling current is a function of the relative orientation of the magnetizations between the two electrodes. By using electrodes that respond differently to magnetic fields, the relative orientation of the magnetizations can be controlled by an external magnetic field of appropriate strength. Generally, the tunneling current is the largest when the electrodes are arranged in parallel, and the tunneling current is the smallest when the electrodes are arranged in antiparallel. MTJs are particularly suitable for use as memory cells in non-volatile memory arrays, such as MRAM, and as magnetic field sensors, for example, in magnetoresistive read heads for magnetic recording disk drives.

The signal-to-noise ratio is very important to the performance of MTJ device applications. The signal magnitude is primarily determined by the magnetoresistance (MR) ratio ΔR/R exhibited by the device, where ΔR is the difference in resistance between the two magnetic configurations. Defining the signal as a voltage output, the magnitude of the signal is given by Ib × ΔR, where Ib is the constant bias tunneling current flowing through the device. Regarding noise, the noise level increases with increasing device resistance R. Therefore, to achieve the best performance of MTJ devices, a large MR ratio and small device resistance are important. How the magnitude of the former is related to the spin polarization of the ferromagnetic electrode and how the magnitude of the latter is related to the properties of the insulating barrier will be described below.

High MR ratios require highly spin-polarized electrode layers. The relationship between MR and the spin polarization P of the electrode can be described by the following commonly adopted approximation [1].

ΔR/R=2P ₁ P ₂ /(1-P ₁ P ₂ ), (1)

where _P1 and _P2 are the spin polarizations of the top and bottom electrodes in the MTJ device, respectively. Ferromagnetic transition metals Fe, Co and Ni and their alloys are common materials used as spin-polarized electrode layers in conventional MTJs. The maximum spin polarization achievable with these materials is about 50% [2]. Thus, for two electrodes with spin polarization P = 50%, the maximum MR obtainable according to equation (1) is 67%. This can be seen as the fundamental limit of MR in conventional MTJ devices and is comparable to what has been reported so far. The general MR value obtained by MTJ using the aforementioned electrode materials at room temperature is 20-40%, optimally about 60%, although rare. Due to the growing demand for higher MR effects, many efforts have been made to surpass this limit. For example, attempts have been made to use some alternative electrode materials such as so-called half-metallic ferromagnets predicted to have spin polarization close to 100%, but half-metals have proven to be extremely difficult to realize in practice.

The resistance of an MTJ device is mainly determined by the resistance of the insulating tunneling barrier layer, since the resistance of the electrical leads and ferromagnetic electrodes contributes little to the device resistance. Therefore, the barrier layer resistance is also the main source of noise in MTJ devices. Furthermore, the resistance is proportional to the inverse of the device's lateral area, since the current passes vertically through the layer plane. For high density applications such as MRAM arrays, this becomes critical as the signal-to-noise ratio deteriorates with the reduced area of the MTJ cell. MTJ resistance is generally described as resistance R times area A (RA). The RA product of the insulation barrier can be simplified as

where d is the thickness of the barrier and  is the tunnel barrier height (Fig. 1(b)). For clarity, the constant Omitted from the exponent term. Therefore, resistance increases exponentially with both d and [phi], and to reduce MTJ resistance, the barrier thickness and/or barrier height must be made small. For MRAM applications, both signal states of the device need to be detected and RA values of 500-1000 ^Ωμm2 yield acceptable signal-to-noise ratios. On the other hand, for magnetoresistive read head applications, a continuous range of signal states must be detectable and RA values of 10 Ωμm or ^less are required to compete with existing metallic giant magnetoresistive heads. In the prior art, the insulating barrier layer in the MTJ includes aluminum oxide Al ₂ O ₃ . Alumina is a stable oxide insulator that can be made very thin with a high degree of layer continuity. To meet the above RA range, it turns out that the alumina barrier thickness needs to be made ultra-thin, about 1 nm for MRAM and about 0.6-0.7 nm for read head. MR generally decreases at this thickness regime, most likely due to the formation of quantum dot defects and/or fine pinholes in the ultrathin tunnel barrier layers required to obtain these very low RA values. The main reason forcing the alumina barrier thickness in this ultra-thin regime is the large barrier height [phi] of 2.3-3 eV, which is formed together with conventional ferromagnetic electrode materials.

Therefore, for the further improvement of MTJ devices, it is necessary to find ways to increase the spin polarization and reduce the barrier resistance without degrading the MR. Considering the above limitations, it is recommended to deviate from the conventional MTJ structure as an appropriate practice.

Contents of the invention

The present invention is a magnetic tunnel junction, wherein the aluminum oxide tunneling barrier layer in the prior art is replaced by a tunneling barrier layer made of a ferromagnetic semiconductor with a lower potential barrier height and spin filtering function. Since spin sensitivity is thereby introduced into the barrier layer, this allows one of the prior art ferromagnetic electrodes to be replaced by a non-magnetic one. An MTJ device comprising such a spin filter barrier with a low effective barrier height ensures enhanced MR effect with tunable resistance and simpler MTJ device results. While the invention has been summarized above, it is defined by the appended claims 1-10.

For a further understanding of the above features and additional features of the present invention, please refer to the following detailed description taken in conjunction with the accompanying drawings.

Description of drawings

Figure 1a shows a cross-sectional view of a conventional MTJ device;

Figure 1b shows the corresponding energy diagram for the tunneling barrier of the MTJ device shown in Figure 1a;

Figure 2a shows a cross-sectional view of a spin filter barrier MTJ device according to the present invention;

Figure 2b shows the corresponding energy diagram for the spin filter barrier MTJ device shown in Figure 2a;

FIG. 3 shows the calculated polarization efficiency as a function of energy splitting of the spin filter barrier in the MTJ device shown in FIG. 2 . In the calculations, a fixed barrier height [phi]=1 eV was used and the polarization efficiencies were calculated for three different barrier thicknesses d=1, 2 and 3 nm, respectively.

FIG. 4 shows the calculated polarization efficiency as a function of energy splitting of the spin filter barrier in the MTJ device shown in FIG. 2 . In the calculations, a fixed barrier thickness d=2 nm was used and the polarization efficiencies were calculated for three different barrier heights [phi]=0.5, 1 and 1.5 eV, respectively.

Detailed ways

Due to the confined spin polarization of the electrodes and the high RA of the alumina barrier, conventional MTJ devices offer little room for further improvement. In particular, many efforts have been made to develop effective methods to reduce the thickness of the alumina barrier to an ultra-thin state while maintaining barrier uniformity. This has been shown to be very difficult. The present invention includes an alternative type of MTJ device structure that has the potential to provide higher spin polarization at reduced RA values compared to conventional MTJ devices.

FIG. 1( a ) shows a cross-sectional view of a prior art MTJ device structure. The bottom ferromagnetic electrode layer (the "pinned" layer), mostly Co, is typically grown on an antiferromagnetic layer (not shown) such as CoO, which determines the permanent magnetization of the bottom ferromagnetic electrode by an exchange bias direction. The purpose of this is to make the bottom electrode insensitive to externally applied magnetic fields. On the other hand, the top electrode ("free" layer) is made of a soft magnetic material such as permalloy (NiFe), so that its magnetization direction can be easily changed by an external magnetic field. In this way, the relative magnetization orientation between the two layers can be controlled. The barrier comprises in most cases a thin layer of amorphous aluminum oxide. Electrical leads are connected to the lower and top electrode layers through which current flows vertically. The MR effect in this device manifests itself as a change in resistance as a function of the relative magnetization orientation between the top "free" layer and the bottom "pinned" layer.

Fig. 2(a) shows a cross-sectional view of the MTJ device structure of the present invention. The device includes a spin-filtering tunneling barrier sandwiched between a bottom nonmagnetic electrode and a top ferromagnetic electrode. The non-magnetic electrodes are composed of any conductive material and are not limited to metals. The top ferromagnetic "free" layer electrode comprises a soft magnetic material in which the magnetization can be easily manipulated by an external magnetic field. The spin filter barrier material may comprise a wide bandgap semiconductor doped with a metal element that induces ferromagnetism in an intrinsically nonmagnetic semiconductor host crystal. These types of materials are known as dilute magnetic semiconductors. In contrast to conventional MTJ devices, the "pinned" layer is represented by a spin-filtering barrier and the MR effect appears as a change in resistance according to the relative magnetization orientation between the top "free" layer and the barrier. In the following, the properties of the ferromagnetic semiconductor barrier will be described in more detail.

Ferromagnetism in semiconductor crystals is induced by spin-polarized charge carriers between metallic impurities. This results in a spin-dependent energy splitting of the conduction band. In other words, one spin orientation has a lower conduction band edge than the opposite spin orientation. This situation is illustrated by the energy diagram of Fig. 2(b) when a ferromagnetic semiconductor is included in the MTJ device as a barrier layer. In the figure, the barrier with average height  splits into two spin-dependent subbands separated by energy 2δ. Now, the charge carriers that will tunnel from one electrode to the other face two different barrier heights, one for spin up and one for spin down. Since the tunneling process is sensitively dependent on the barrier height, the splitting of the conduction band greatly increases the tunneling probability of spin-up electrons. Unlike the barrier resistance given by equation (2) for the unpolarized barrier, the spin filter barrier resistance becomes split into two spin components

In a similar manner to defining the spin polarization P of ferromagnets [1], the polarization efficiency PB of the spin filter barrier can be written as

${\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; RA</span>}}_{<span\; class="notranslate">\; \&DoubleDownArrow;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; RA</span>}}_{<span\; class="notranslate">\; \&DoubleUpArrow;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; RA</span>}}_{<span\; class="notranslate">\; \&DoubleDownArrow;</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; RA</span>}}_{<span\; class="notranslate">\; \&DoubleUpArrow;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

To estimate the polarization efficiency, the spin filter barrier will be exemplified by a ferromagnetic semiconductor including ZnO as a wide bandgap (Eg=3.2eV) semiconductor host and a ferromagnetism-inducing metal element (ME). This ferromagnetic semiconductor will be referred to as ZnMEO hereinafter. Other magnetic semiconductor materials may also be used.

3-4 show the polarization efficiency PB calculated using Equation 4 as a function of energy splitting 2δ for various barrier parameters. In Figure 3, the barrier height is fixed at 1 eV, which represents a typical barrier height between a metal contact and a wide bandgap semiconductor, and the barrier thickness d is varied between 1 and 3 nm. In Fig. 4, the barrier thickness d is fixed at 2 nm, and the barrier height  varies between 0.5 and 1.5 eV. Briefly summarizing the results of Figures 3 and 4, the polarization efficiency increases with increasing barrier thickness and decreasing barrier height. The actual value of energy splitting in ZnMEO depends on the type and doping level of ME used. No reported values are currently available due to the new discovery of room temperature ferromagnetism in these types of materials. However, the well-studied insulator EuS becomes ferromagnetic at low temperatures and thus represents a similar material class to ZnMEO. In EuS, the spin-dependent energy splitting of the conduction band is 360meV [5]. Assuming that the energy splitting in ZnMEO is only half that of EuS, i.e. 180eV, and using a barrier height of 1eV, for a 2nm thick ZnMEO spin-filter barrier, according to Fig. 3, the polarization efficiency is about 73%. To estimate the MR exhibited by the invention implemented in FIG. 1, refer to Equation 1. In contrast to conventional MTJs, the present invention uses a non-magnetic bottom electrode and the spin sensitivity is introduced in the barrier layer. Therefore, the term P2 in Equation 1 is replaced by the spin filter efficiency PB. Using PB = 73%, from the previous estimates, and for a highly spin-polarized top electrode P1 = 50%, an MR ratio of 115% is obtained.

The predicted MR ratios of over 100% for the spin filter devices of the present invention greatly exceed the highest reported MR ratios (up to 60%) for conventional MTJ devices. Furthermore, since the tunneling barrier realized in Figure 2 includes a wide bandgap semiconductor, exemplified by ZnMEO with a bandgap of 3.2eV, the resistance-area (RA) product of the device is inherently lower than that of the alumina insulator used in the prior art . In this way, ultra-thin barrier thickness conditions are avoided. It is estimated that the ZnMEO barrier will exhibit an RA value matching that of alumina at a thickness more than twice that of the alumina barrier. This estimate is supported by a recent report on barrier layers of ZnSe, another wide bandgap semiconductor similar to ZnO, with a bandgap of 2.8eV [6]. Thus, the present invention implemented in Figure 2, with the features previously described with reference to Figures 3-4, meets the need for improved signal-to-noise ratio in improved MTJ device applications such as MRAM arrays and magnetoresistive read heads. Other synergistic effects of the present invention will be described below.

The magnetic field strength (coercive force) required to reverse the magnetization direction in ferromagnetic semiconductors such as ZnMEO is typically about two orders of magnitude greater than permalloy typically used as the top electrode "free" layer in MTJs. This indicates that the spin filter barrier layer in the present invention does not need to be magnetically biased by the underlying antiferromagnetic layer, as is the case with the bottom electrode "pinned" layer in conventional MTJ devices. This greatly simplifies the MTJ device structure. Furthermore, the use of a non-magnetic bottom electrode opens up the choice of conductive materials as compared to prior art ferromagnetic bottom electrodes. This includes metallic conductors such as Cu, Al or Au, as well as degenerate semiconductors. For example, using n-type Si as the bottom electrode provides important compatibility with Si processes and CMOS technologies in a straightforward manner. Many reports have demonstrated good quality thin continuous ZnO films on Si wafer substrates by various deposition techniques. Another example offers the very attractive possibility of epitaxial ZnMEO barrier layer by using degenerate ZnAlO as the bottom electrode layer. ZnAlO is a semimetal often used as a conductor in solar cell applications and has a very good crystal match with ZnMEO.

references

[1] M. Julliere, Phys. Lett. 54A, 225 (1975)

[2] R. Meservey, and P.M. Tedrow, Phys. Rep. 238, 173 (1994)

[3] Y.Ji, G.J.Strijkers, F.Y.Yang, C.L.Chien, J.M.Byers, A.Anguelouch, G.Xiao, and A.Gupta, Phys.Rev.Lett.86, 5585(2001)

[4] W.E.Pickett, and J.S.Moodera, Phys.Today 5, 39(2001)

[5] A. Mauger, and C. Godart, Phys. Rep. 141, 51 (1986)

[6] X.Jiang, A.F.Panchula, and S.S.P.Parkin, Appl.Phys.Lett.83, 5244(2003)

Claims (10)

1. the magnetic tunnel-junction with tunneling barrier layer is characterized in that described tunneling barrier layer comprises the dilute magnetic semiconductor with spin sensitivity.

2. magnetic tunnel-junction as claimed in claim 1 comprises the end lead-in wire that is coupled to hearth electrode, and this hearth electrode is coupled to dilute magnetic semiconductor, and this dilute magnetic semiconductor is coupled to top electrode, and this top electrode is coupled to the top lead-in wire, it is characterized in that described hearth electrode is non-magnetic.

3. magnetic tunnel-junction as claimed in claim 2 is characterized in that, described hearth electrode comprises n type Si.

4. magnetic tunnel-junction as claimed in claim 2 is characterized in that described hearth electrode comprises degeneracy ZnAlO.

5. magnetic tunnel-junction as claimed in claim 1 is characterized in that described tunnel junction comprises the spin filter element, and it has magnetoresistance (MR) ratio above 60%.

6. magnetic tunnel-junction as claimed in claim 1 is characterized in that, described dilute magnetic semiconductor is the wide band gap semiconducter that surpasses 2.7eV.

7. magnetic tunnel-junction as claimed in claim 6 is characterized in that described dilute magnetic semiconductor comprises ZnMEO.

8. parts is characterized in that it comprises each described magnetic tunnel-junction according to claim 1-6.

9. parts as claimed in claim 8 is characterized in that it is embodied as in the following parts any: non-volatile MAGNETIC RANDOM ACCESS MEMORY (MRAM), the magnetoresistance read head that is used for disc driver, Spin Valve/magnetic channel transistor, ultrafast optical switch, have luminescent device, the logic processing device of polarization modulation output.

10. computer, it is characterized in that it comprise according to claim 1-6 each magnetic tunnel-junction and/or each parts according to Claim 8-9.

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